Despite a variety of mechanised means now available for detecting and clearing landmines, the current hand tool of choice is the hand prodder. Personnel exhibit greater confidence when traversing a minefield which has been hand-prodded by their compatriots than they do with fields cleared by other means.
The hand prodder typically comprises a 30 cm long pointed rod extending from a gripping handle. The probe is generally non-magnetic to avoid setting off magnetically-triggered mines. The user probes the ground ahead and excavates any hard objects which the probe contacts. As the ratio of rocks to landmines in a minefield may number 1000:1, excavation of every contact is labourious, but very necessary.
Thus, it would be a significant advance in the art should a user be able to discriminate between landmines and rocks upon contact and without excavation. Accordingly, applicant sought to provide a hand probe which could distinguish variations in the object's material characteristics.
As shown in FIG. 1, known apparatus for measuring the compressive characteristics of materials include the Split Hopkinson Pressure Bar apparatus ("SHPB"). The SHPB is typically used to apply rapid strain rates (100,000 mm/mm/second) to samples; compressing them for the measurement of mechanical properties. A sample is placed between the ends of two axially aligned elastic bars. Maintaining elastic conditions in the bar, the first "incident" bar is struck, rapidly compressing the sample between the incident bar and the second "transmitter" bar.
The act of striking the incident bar sends a high frequency elastic mechanical pulse or compression wave through the bar. Like an acoustic wave, it reflects from interfaces having differing characteristics. Dependent upon the samples material characteristics, a portion of the wave reflects from the incident bar/sample interface and travels back along the incident bar. The remainder of the wave passes through the sample. A lesser reflection occurs at the transmitter bar/sample interface. The residual portion of the wave continues as a compression wave along the transmitter bar.
Strain gauges located on both the incident and transmitter bars enable calculation of the strain in the bars. In the incident bar, the displacement of the bar's end is proportional to the sum of the strain in the bar which is calculated from time-shifted strain gauge data obtained for both the incident and reflected waves. The displacement of the transmitter bar end is proportional to the strain measured in the transmitter bar. The sum of the displacements of the ends of the incident and transmitter bars defines the compression of the sample.
Mechanical impedance ("MI") is a material's characteristic which relates to the material's effect on acoustic wave transmission and reflection. Not surprisingly, MI affects the nature of the reflected wave in the SHPB's incident bar. The effect of MI on the SHPB apparatus, for materials having differing MI values, is described for three particular cases as follows.
In a first case, following the basic rules of mechanics of materials, if the MI of the sample is the same as that of the bar, then there is no reflective interface and thus no wave reflection at all; the sample is elastically displaced exactly as is the bar itself. The displacement at the bar's end is directly proportional to the measured strain (.epsilon.).
If MI of the sample is very much greater than that of the bar, then the sample's MI is effectively infinity, and all of the incident wave is reflected. The incident and reflected waves are also in phase. The reflected wave is therefore also compressive and equal in magnitude to the incident wave. Thus the resultant bar end displacement is zero.
If the MI is zero (no sample at all, unconstrained bar end), the reflected wave is tensile, but of equal magnitude to the incident wave. The phase of the wave shifts 180.degree. and is thus out of phase. In other words, the net stress cancels and the relative displacement at the bar end equals twice that for the first case (2.times..epsilon.).
In tabular form, the above cases and the general case are shown as:
______________________________________ Strain Strain Proportional MI Incident Reflected Displace Case Sample .epsilon..sub.i = .epsilon..sub.r = (.epsilon..sub.i -.epsilon..sub.r) ______________________________________ 1 = bar .epsilon..sub.i 0 .epsilon..sub.i 2 .infin. .epsilon..sub.i .epsilon..sub.i 0 3 0 .epsilon..sub.i -.epsilon..sub.i 2 .times. .epsilon..sub.i General ? .epsilon..sub.i .epsilon..sub.r (.epsilon..sub.i - .epsilon..sub.r) ______________________________________
Knowing the relative displacements of the bars, the displacement imposed on the sample is also known. From the Young's Modulus (E) and the displacement of a bar, the imposed stress is also known. The force imposed is equal to the product of the stress and bar's cross-sectional area. Thus the strain and stress functions as they apply to the sample may also be determined.
As the loading on the sample substantially equalizes after a very short time, it is known to make a simplifying assumption and merely apply the strain results for either one of the incident bar or the transmitter bar. In another arrangement, the striker is permitted to impact directly on the sample, and the transmitter bar results alone are used to define the sample characteristics.
The question is, can such an approach be successfully applied to materials as diverse as plastics, minerals and metals and enable one to sort out non-hazardous from the potentially hazardous prodder contacts.